Plan B, also called emergency contraception, prevents unwanted pregnancies after unprotected sex or if the first form of birth control, like a condom, fails. The hormone in Plan B, levonorgestrel, keeps the ovaries from releasing an egg. Drugs.com points out that Plan B thins the uterine lining, which prevents the egg from implanting; the drug also thickens cervical mucus, which keeps the sperm from reaching the egg. Plan B is not 100 percent effective, so a pregnancy after taking Plan B may occur.

Time Frame

Planned Parenthood explains that women can take Plan B up to 120 hours after sexual intercourse. But the sooner a woman takes Plan B, the more effective the drug is. The McKinley Health Center at the University of Illinois notes that Plan B is 95 percent effective when taken within 24 hours; the effectiveness drops to 61 percent if taken 48 to 72 hours after intercourse.

Proper Use

Drugs.com notes that women should take Plan B as soon as they can. With the Plan B version that contains two pills, women take the second pill 12 hours after the first. Plan B One-Step only has one pill. Since Plan B can cause nausea, Planned Parenthood recommends using an anti-nausea medication an hour before to prevent vomiting.

Effects on Existing Pregnancy

Kristi Monson, PharmD, and Arthur Schoenstadt, MD, authors of the eMedTV article "Plan B and Pregnancy," point out that Plan B will not terminate an already existing pregnancy. Since the hormones in Plan B affect ovulation, the cervical mucus and the uterine lining, it cannot cause an abortion. Women who know they are pregnant should not use Plan B.

Considerations

Specific factors can increase the likelihood of becoming pregnant after taking Plan B. For example, the longer a woman waits to take Plan B after having unprotected sex, the higher her risk is of becoming pregnant. Planned Parenthood notes that vomiting after taking Plan B decreases the effectiveness of Plan B, as the body does not fully absorb the medication.

Warning Signs

Certain signs can indicate that Plan B did not work. For example, Drugs.com explains that a late period, which is a week or more late, may indicate a pregnancy. Some women may have other signs of a pregnancy. Vanessa Cullins, MD, MPH, MBA, of Planned Parenthood lists sore or enlarged breasts, frequent urination, nausea, fatigue and headaches as signs of a pregnancy.

Effects on Fertility

The Feminist Women's Health Center notes that Plan B does not interfere in a woman's ability to become pregnant in the future. Other forms of birth control, such as condoms or birth control pills, are for long-term use and can protect against unwanted pregnancies.